1. Field of the Invention
The invention relates to dynamically vulcanized thermoplastic compositions having improved flame retarding properties and a process for preparing the compositions.
2. Description of Information Disclosures
In recent years, there has been a significant commercial interest in polymer blends which have a combination of both elastic and thermoplastic properties. These polymer blends have been given the generic designation of Thermoplastic Olefins ("TPO"). They exhibit some of the properties of a cured elastomer as well as the reprocessability of thermoplastic resins. The elastomeric characteristics are enhanced if one component of the blend is a vulcanizable elastomer which is wholly or partially crosslinked.
The earliest work in the curing of TPO compositions was by Gessler and Haslett; see U.S. Pat. No. 3,037,954. That patent teaches the concept of "dynamic curing" wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer cured while continuously mixing and shearing the polymer blend. The result is a micro-gel dispersion of cured rubber in an uncured matrix of resinous thermoplastic polymer. Gessler's U.S. Pat. No. 3,037,954 discloses compositions comprising polypropylene and a rubber wherein the rubber may be, butyl rubber, chlorinated butyl rubber, polybutadiene, polychloroprene and polyisobutene. Compositions of about 50 to 95 parts polypropylene and about 5 to 50 parts of rubber are disclosed.
U.S. Pat. No. 3,081,279 discloses compositions wherein a rubber, such as butyl rubber, or halogenated butyl rubber is compounded with a halosulfonated polyolefin and N-methyl-n-nitroso p-nitroso aniline. Subsequently, the compounded rubber is cured, preferably in a mold.
U.S. Pat. No. 2,369,471 disclosed blends of ethylene polymers with various hydrocarbon rubbers and halogenated rubbers which may be halogenated ethylene polymers. The compositions are uncured or, if desired, the blends may be further subjected to vulcanization employing known vulcanizing agents.
U.S. Pat. No. 4,141,878 discloses a thermoplastic elastomeric composition comprising a polyolefin resin and a crosslinked chlorosulfonated polyethylene rubber.
U.S. Pat. No. 4,286,079 discloses a curable composition comprising a chlorinated polyethylene, a sulfur curable rubber, which may be brominated butyl rubber, with or without a vinyl chloride resin, sulfur with or without a sulfur donor and at least one amino acid. The composition may be used in roofing sheets. See also U.S. Pat. No. 4,262,098.
When it is desired to use elastomeric thermoplastic compositions for roofing sheets, it is important to increase the flame retarding properties of these compositions.
It has now been found that the flame retarding properties of elastomeric thermoplastic compositions can be improved by dynamically vulcanizing a blend of specified polymers.